The present invention relates to an optical alignment system for radiography.
Although the clinical importance of portable x-ray examinations is beyond question, the image quality may be inferior to that obtained with fixed radiograph apparatus in an x-ray department. This inferior image quality is commonly attributed to intrinsic limitations of portable radiography apparatus, however, it is in fact due mainly to uncontrolled scattered radiation which fogs the radiograph, thus reducing contrast and obfuscating diagnostic information.
Use of an accurately aligned anti-scatter grid can provide consistently high image quality, but precise alignment of the grid relative to the x-ray source is important for good results.
For portable radiography, good results can be achieved with a 6:1 or 8:1 anti-scatter grid, provided that the x-ray beam energy is no greater than 90 KV, and provided that the anti-scatter grid is accurately aligned with respect to the x-ray source. Referring to FIG. 1, accurate alignment of a radiographic machine 20 relative to anti-scatter grid 21 and x-ray image medium 22 is illustrated in position A while inaccurate alignment is illustrated in position B.
In the case of a conventional lead strip linear anti-scatter grid, alignment is critical only in the dimension across the grid lines. Moderate angulation error along the direction of the grid lines does not significantly impair image quality. In other words, referring again to FIG. 1, moderate misaligmnent about a horizontal axis lying within the plane of the page is not so critical, whereas alignment about an axis perpendicular to the plane of the page is important.
Further, with a focused anti-scatter grid, it is also desirable for central x-ray beam 23 of x-ray machine 20 to be centered accurately with respect to anti-scatter grid 21. Further, variations in focus distance when using a focused grid are important, but mainly affect film density provided that the x-ray machine 20 is properly centered and aligned relative to anti-scatter grid 21. In the case of a two dimensional grid, for example a cross-hatched grid or pinhole grid, alignment is also important in two dimensions.
An approach that addresses this alignment problem is presented in U.S. Pat. No. 4,752,948 issued Jun. 21, 1988, and assigned to the same assignee as the present application. The disclosure of this patent is expressly incorporated herein by reference. While adequately addressing the problem of alignment between the x-ray beam and anti-scatter grid in a portable x-ray apparatus, this patented device presents a mechanical system which has proven somewhat difficult to retrofit to existing portable x-ray apparatus, or to use with very ill patients who are unable to cooperate.
Another system that addresses this alignment problem is presented in U.S. Pat. No. 5,241,578, issued Aug. 31, 1993, and assigned to the same assignee as the present application. The disclosure of this patent is also expressly incorporated herein by reference. This device employs a laser device mounted to the collimator housing of a portable x-ray machine with a mirror reflector on the grid cassette, thus eliminating any mechanical connection between the x-ray machine and the grid cassette. However, modification of the x-ray unit to incorporate the laser device is required, which, once again, renders the device somewhat difficult to retrofit to existing portable x-ray machines.